This invention relates to water gel explosive compositions and particularly to incorporating therewith sensitizing amounts of certain amine nitrate sensitizers and an aluminum sensitizer having from about 3 to about 9 square meters of surface area per gram. In another aspect, this invention relates to an improved water gel explosive composition wherein the sensitizing effects of paint grade aluminum are significantly enhanced by employing relatively small amounts of certain amine nitrate sensitizers. In a further aspect this invention relates to a process for producing water gel explosive compositions sensitized with a combination of paint grade aluminum and certain amine nitrate sensitizing agents.
Modern water gel cap sensitive explosive compositions are, in some cases, sensitized by addition of some type of an alkyl or alkanol amine nitrate. Exemplary of an alkyl amine nitrate component is methylamine nitrate, which has, heretofore, been incorporated in relatively large weight percentage amounts in the water gel explosive composition in order to achieve sufficient sensitivity. Typically, at least between about 30 to about 40 weight percent of methylamine nitrate or similar amine nitrates has been required in order to produce a cap sensitive water gel explosive. Entrained air bubbles or microballoons must also be added to achieve the desired sensitivity. However, even with the use of such large quantities of methylamine nitrate, it has been found difficult to obtain low temperature detonatability below about 40.degree. to about 50.degree. F. Explosives having low temperature detonation characteristics are required for use in numerous applications, such as underground mines, and above ground applications, such as ditching operations in colder climates. Further, it is difficult to obtain water gel explosives which meet the air gap test standards (as further defined hereinafter) so as to be suitable for use as "permissibles" in mining operations even if relatively large amounts of amine nitrate sensitizers are employed.
In order to provide a suitable amine nitrate component content in the explosive composition, highly concentrated water solutions of the amine nitrate component are required. Recent studies have shown that concentrated water solutions having greater than approximately 65 weight percent or more of the amine nitrate component are very sensitive to mechanical handling and are easily detonated by relatively low mechanical impulses. Thus, preparation of water gel cap sensitive explosive compositions, sensitized solely by large amounts of an amine nitrate component, can be extremely dangerous unless performed with care. An example of water gel explosives containing relatively large quantities of amine nitrates are the explosives described in U.S. Pat. No. 3,431,155, issued Mar. 4, 1969.
Water gel cap sensitive explosive compositions have also been sensitized by other materials such as paint grade aluminum. These gels often contain from about 3 to about 6 weight percent paint grade aluminum. This sensitizer can be a very effective sensitizer, but it is difficult to handle because of severe dusting problems. Recently nondusting types of paint grade aluminum have been developed which partially alleviate these handling problems. However, there are limits on the maximum achievable sensitivity, and explosive compositions containing paint grade aluminum are not sufficiently sensitive for certain important applications. In this regard, it is difficult to achieve adequate cap sensitivity at temperatures at or below 40.degree. or 50.degree. F even when up to about 7.5 percent paint grade aluminum is employed. Finally, paint grade aluminum sensitized water gel explosive compositions often have difficulty in detonating across a three inch air gap in the standard half cartridge test. This test is required by the Bureau of Mines for explosives which are to be employed in underground coal mining applications. Basically, the test requires that one half of a cartridge of explosive be able to detonate a second half across an air gap of at least 3 inches. Explosive compositions passing this test, and the other requirements of the Bureau of Mines, are referred to as "permissibles" in that use of the explosives is approved and permitted in underground coal mining procedures. The term "permissible explosives" as used herein is defined to mean explosive compositions which meet the standards set forth by the Bureau of Mines in 30 C.F.R. 15, et seq. These standards include the above described air gap test as well as other requirements including, composition tolerances, propagation characteristics, gallery test requirements, poisonous gas production limits, and friction tolerances. Interestingly, the air gap sensitivity characteristics necessary to pass "permissible" standards and low temperature detonatability cannot be significantly improved by increasing the amount of paint grade aluminum in the water gel explosive compositions.
In summary, although amine nitrate components as well as paint grade aluminum have been incorporated into water gel explosive compositions, these compositions have proved inadequate or marginal for certain, important field applications. Thus, U.S. Pat. No. 3,962,001 discloses explosive compositions having improved low temperature detonation characteristics which comprise hexamethylenetetramine and paint grade aluminum as well as nitric acid. However, such compositions must be prepared following critical process conditions which limit their commercial desirability. The present invention has been provided to remedy these deficiencies as the explosive compositions disclosed herein exhibit excellent low temperature detonability, and air gap sensitivity.